1. Technical Field
The present disclosure relates to systems and methods for providing energy to biological tissue and, more particularly, to improved apparatus and methods for amplifying microwave energy for use during surgical procedures.
2. Background of Related Art
Energy-based tissue treatment is well known in the art. Various types of energy (e.g., electrical, ultrasonic, microwave, cryogenic, thermal, laser, etc.) are applied to tissue to achieve a desired result. Electrosurgery involves application of high radio frequency electrical current to a surgical site to cut, ablate, coagulate or seal tissue. In monopolar electrosurgery, a source or active electrode delivers radio frequency energy from the electrosurgical generator to the tissue and a return electrode carries the current back to the generator. In monopolar electrosurgery, the source electrode is typically part of the surgical instrument held by the surgeon and applied to the tissue to be treated. A patient return electrode is placed remotely from the active electrode to carry the current back to the generator.
In tissue ablation electrosurgery, the radio frequency energy may be delivered to targeted tissue by an antenna or probe. In this instance, a high radio frequency electrical current in a microwave range of about 900 MHz to about 5 GHz is applied to a targeted tissue site to create an ablation volume, which may have a particular size and shape. Typically, microwave apparatus for use in ablation procedures include a microwave generator, which functions as an energy source, and a microwave surgical instrument having an antenna assembly for directing the energy to the target tissue. The microwave generator and surgical instrument are typically operatively coupled by a cable assembly having a plurality of conductors for transmitting microwave energy from the generator to the instrument, and for communicating control, feedback and identification signals between the instrument and the generator.
The microwave generator commonly includes a microwave oscillator coupled to a power amplifier. The microwave oscillator generates a relatively low-power surgical signal that is amplified by a microwave amplifier to produce a signal of sufficient power to achieve the desired effect, e.g., tissue ablation. A user, typically a surgeon, may specify a particular output level, which may be accomplished by varying the amplitude of the relatively low-power input surgical signal to the microwave amplifier. With decreasing input levels, an amplifier operates in linear mode where efficiency decreases, e.g., thermal power dissipation increases. Conversely, with increasing input levels, an amplifier operates at or near saturation mode where maximum efficiency is achieved and thermal power dissipation is at a minimum.
Commonly used microwave power amplifiers are known to be inefficient. For example, a class AB microwave power amplifier typically exhibits an efficiency of about 35%. That is, to achieve a surgical signal of 250 W, a class AB power amplifier requires about 714 W of power, of which 464 W is dissipated as thermal energy. The resulting heat becomes difficult to manage and may require the use of bulky and costly cooling systems, e.g., fans and heat sinks. Additionally, the excess heat may cause thermal stress to other components of the generator, shortening generator life, decreasing reliability, and increasing maintenance costs.
Additionally, a class AB amplifier may exhibit crossover distortion that introduces undesirable harmonics into the surgical signal, which are known to cause radiofrequency interference in excess of acceptable limits.